Magnetic structures of TbMn6Sn6 and HoMn6Sn6 compounds from neutron diffraction study

Neutron diffraction measurements have been performed on the ternary stannides TbMn₆Sn₆ and HoMn₆Sn₆ of HfFe₆Ge₆ type structure (space group, P6/mmm). This structure can be described as a filled derivative of the CoSn B35-type structure. Each of the rare earth and manganese atoms is successively distributed in alternate layers stacked along the c axis with the sequence Mn-R-Mn-Mn-R-Mn. Owing to the manganese atom coordination of the rare earth, this structure appears closely related to the CaCu⁶- and ThMn₁₂-type structures. This study confirms that both the rare earth and the manganese sublattices order simultaneously above room temperature. In the whole temperature range studied (2–300 K), TbMn₆Sn₆ and HoMn₆Sn₆ exhibit a collinear ferrimagnetic arrangement. At 300 K, the magnetic structure consists of a stacking of ferromagnetic (001) layers of rare earth and manganese with the coupling sequence Mn(+)R(−)Mn(+)Mn(+)-R (−)Mn(+) (μ_Mn ≈ 2.0(1)μ_B, μ_Tb = 4.9(1)μ_B, μ_Ho = 4.7(1)μ_B). For HoMn₆Sn₆, the magnetic moments lie in the (001) plane while in TbMn₆Sn₆, they deviate by φ = 15° from the c axis. Below 300 K, the two compounds exhibit a spin rotation process and at 2 K the magnetic moments are along [001] and at 50° from this axis for TbMn₆Sn₆ and HoMn₆Sn₆ respectively μ_Mn = 2.4(1)μ_B, μ_Tb = 8.6(1)μ_B, μ_Ho = 8.4(1)μ_B).

The results are discussed in terms of Ruderman-Kittel-Kasuy-Yoshida exchange interactions and compared with those obtained previously for the parent RFe₆Al₆ and RCo₆ compounds.